While your character’s class represents a specialization, and generally defines how they approach combat, skills model their general body of knowledge and supplementary abilities that many adventurers have in common. Two rangers with the same tracks may play very differently if one focuses on sneaking around and sabotaging things, and the other relies on sharp senses and a vast knowledge of different creatures to bring down his enemies.
How to Use Skills
To make a skill check, roll: 1d20 + skill modifier. Your skill modifier for a given skill is equal to skill rank + ability modifier + any miscellaneous modifiers. This roll works just like an attack roll or a saving throw – the higher the roll, the better. Either you're trying to match or exceed a certain Difficulty Class (DC), or you're trying to beat another character's check result.
Generally speaking, skill checks in your campaign should require a d20 roll of 10 to 12 for an average character who's trained that skill. If the GM wants to implement a particularly challenging situation, the DC might require a result of 14-17 for an average character who's trained that skill. For example, a DC 15 challenge is a fairly reasonable typical challenge for a 1st- or 2nd-level character (1 or 2 ranks +3 stat means that the character needs a 10 or 11 to succeed). A DC 20 challenge is quite difficult for a 2nd-level character, but fairly typical for a 4th-level character. A DC 30 check is a serious challenge for a typical 10th-level character, and even a 12th-level character who is heavily optimized for a specific skill will find a DC 35 check difficult to reliably make.
In each skill entry, some tasks are given, with a normal DC for achieving that task. The game is written and balanced around these DCs; however, no list of tasks could be exhaustive and the GM should use his or her discretion in deciding where a given task falls in the DC range. Beyond DC 35, we have not consistently provided representative tasks, but GMs should use imagination to come up with suitably epic challenges at higher levels. It should be noted that without a substantial focus in a specific skill and the aid of party members, even a 20th-level character will not be able to reach DC 50 with any degree of reliability, so obviously DCs should generally cap out at 50 for the most absurd extrapolation of a skill that you can imagine.
We generally place a greater value on speed and simplicity in terms of skill use than on the strict simulation of a challenge. As a result, we tend to prefer an environment where the GM says "Make an Athletics check to climb the cliff" to an environment where the GM says "Okay, your turn, make a Climb check to move this round." For this reason, we suggest that in situations where the PCs are dealing with primarily environmental hazards (as opposed to rolling against an opponent), skill checks of the same skill should be kept down to one per challenge, or two or three if there are logically different steps to the challenge (such as picking a lock, then disarming a trap, then opening a safe, all of which are Larceny checks). Notes regarding this general principle are placed in specific skill descriptions as an aid to GMs.
Trained Skills and Skill Ranks
You either have a skill trained, or you don't. You choose trained skills at 1st level, as covered in the Quick Start Guide or Advanced Character Creation. Trained skills have a skill rank equal to your level; untrained skills have no rank. A character's number of ranks in a skill is equal to the character's level. Many skills can be used even if the character has no ranks in them; doing this is called making an untrained skill check.
The ability modifier used in a skill check is the modifier for the skill's key ability (the ability associated with the skill's use). The key ability of each skill is noted in its description.
Miscellaneous modifiers include racial bonuses, bonuses from active magical effects or items, and bonuses provided by feats, among others.
All knowledge skills have a primary function identifying creatures, objects, and devices. Each knowledge skill represents a general category, something like an academic department at a university. Certain feats and class features allow you to use successful knowledge checks to gain mechanical bonuses against your opponents in a combat, in addition to the tactical advantages of knowing a monster's mechanical strengths and weaknesses. Finally, each knowledge skill has specific uses in its general “field” that can affect the game world.
- Identify Creature: As a swift action, you can make a Knowledge check to identify a creature. The DC for identifying a creature is equal to 10 + the creature's level; a successful check gives you a general idea of the creature's special powers. For each 5 points by which your Knowledge check exceeds the DC, you can ask a specific question about one of the creature's defenses or special attacks (such as a numerical value, or which defense is weakest).
- Identify Ability: You may identify an ability immediately after you see it used by making a Knowledge check (DC 10 + the level of creature that used the ability). This does not require an action. [Legendary] abilities add +2 to the DC.
The Knowledge skill required for this check depends on the type of ability it is used to identify. Extraordinary abilities and abilities granted by a race, [Racial] feat, or racial track are identified with the Knowledge skill normally used to identify creatures of the user's type. Spells, supernatural, and spell-like abilities are identified with the Arcana skill.
Interaction skills cover your character’s ability to interact with other creatures and secure their cooperation (or at least reduce their ability to thwart your goals). In a combat encounter, each skill allows you to distract or counter opponents; in social encounters, each skill allows you to gain advantages in a negotiation, interrogation, or similar adversarial setting.